1. Field of the Invention
The present invention relates to an electric motor, and more particularly to an electric asynchronous motor and method of using an electric asynchronous motor.
2. Description of the Related Art
DE 299 08 433.7 discloses an electric motor for a drive of a roll or cylinder of a paper making or a paper finishing machine. This device may be used as a direct drive or as an indirect drive. “Direct drive” means to directly connect the motor's rotor to a journal of a roll or cylinder, without the use of any intermediate power transmission, e.g. a speed reducing gear transmission. The electric motor is designed as a so-called “slip-on motor” and arranged between the body and the bearing of the roll or cylinder. Openings are provided in the motor's housing for the supply of cooling air to the motor's inner elements. An additional blower may be activated, when the driving speed is low.
DE 100 35 578.1 discloses an electric motor designed for directly driving a roll or a cylinder of a paper machine. The cooling system of this motor uses lubricating oil, which is primarily for the lubrication of the bearings of the roll or the cylinder. The cooling system includes a cooling line extending helically around the motor's stator, and more precisely between the stator and the housing.
WO 97/45941 discloses a so-called “solid rotor” for an electric machine. The solid rotor has some advantages over the conventional laminated rotor, but there are overheating problems, particularly in cases where small rotational speeds are desired, which are needed for a direct drive. WO 97/45941 seeks to overcome the overheating problem by way of some different types of solid rotors, all having at their end faces short-circuit rings of a material having good electrical conductivity. However, an electric machine using any of the rotors disclosed in WO 97/45941 still needs a cooling system, particularly if it is utilized for a direct drive, e.g. for rolls or cylinders of paper machines. WO 97/45941 does not disclose any manner cooling at all.
DE 199 05 538 A1 discloses an electric motor for the drive of a vehicle. The electric motor includes an annular rotor and an annular stator, both being positioned in a housing. A cooling system is included having a heat exchanger for transferring heat from the rotor and the stator via an internal cooling air circuit to a cooling liquid. The heat exchanger is positioned outside a part of the housing section, which is wrapped around the stator. The internal cooling air circuit includes an air stream, which flows through the heat exchanger and through an area between the rotor and a solid motor shaft. The solid motor shaft connected to the rotor by way of connecting elements, e.g. radial vanes. The capacity of this cooling system may be sufficient, if the motor is utilized to drive a vehicle. However, if the motor were used to drive any element of a paper machine, particularly in a hot environment and as a direct drive, then the capacity of the cooling system would be insufficient.
Also, in the electric motor disclosed in DE 199 05 538 A1, the ratio between the outer diameter of the rotor and the outer diameter of the housing section, which is directly wrapped around the stator, the ratio is about 0.5:1, which is a relatively low value resulting in low torque, particularly at low speed. This is another reason why this known electric motor is useless for the direct drive of a machine component such as a roll or a cylinder of a paper machine.
What is needed in the art is an electric motor that is adequately cooled even at slow speeds.